Impact of Induction Temperature on Rolling Contact Fatigue in Laser Solid-State Phase Transformation of U75V
Objective U75V is one of the major rails of heavy railway transportation in China.Owing to increasingly harsh service conditions,wheel-rail rolling contact fatigue(RCF)has become the major failure mode of the wheel-rail system.This requires the rail to have high strength,wear resistance,and good rolling contact fatigue performance.The surface properties of materials are essential in the rolling contact fatigue performance,and the fatigue life can be effectively enhanced by rational surface treatment technology.Since the emergence of laser surface treatment technology in the 1970s,it has become the main means of material surface treatment owing to its wide application range and high processing efficiency.Laser transformation hardening is the most widely used among the various types of laser surface treatments.While this technique significantly improves the surface hardness of treated materials,it causes higher brittleness,which can decrease fatigue performance.This study adopts a new laser solid-state phase transformation process to enhance the surface quality of U75V steel by combining laser and induction as two heat sources.The main focus is the influence of different induction-holding temperature treatments on the rolling contact fatigue performance of the material's strengthening layer.The principle is to regulate the proportion of microstructures in the material by changing the induction holding temperature.This paper analyzes the morphology of damage caused by the wheel-rail rolling contact.It further explores the fatigue damage mechanism of U75V through a finite element simulation.Methods In this study,a sample of the U75V rail material after hot rolling with a geometric size of 300 mm × 70 mm × 30 mm was used as the substrate.Before the experiment,the surface oxide layer and rust stains were removed using pre-grinding and cleaned with alcohol.Moreover,a light-absorbing coating was uniformly applied to the processed surface to improve the laser absorption efficiency of the material.First,laser-induction hybrid solid-state phase transformation was employed to regulate the surface microstructure of the material by changing the induction preheating temperature,and samples with different surface structure ratios were obtained.Subsequently,a rolling contact fatigue testing machine was used to roll the surface of the sample in a reciprocating cycle.After 30× 104 cycles,a wire electrical discharge machine was used to cut the critical sections of the samples for later observation.The surface to be observed was cleaned using an ultrasonic cleaning machine.The damage morphology of different samples was observed using a 3D confocal height measuring instrument(Keyence VK-X1000)and a scanning electron microscope(SEM,EVO18,ZEISS,Germany).Results and discussions The induction temperature has a significant impact on the surface damage morphology of the material after the rolling contact cycle.As the induction temperature decreases,the proportion of martensite structure within the strengthened layer gradually increases,which significantly influences the surface damage morphology of the material,primarily affecting the area(Fig.9)and depth(Fig.10)of surface pitting.The pitting on the substrate sample appears flaky,dense,and irregularly arranged,with an average size of 15-20 μm and a depth of approximately 3.99 μm.After induction treatment at 550 ℃,the amount of pitting on the sample significantly decreases,and the shape of pitting is mostly elongated,with an average size of 5-10 μm,and the depth decreases by 40％compared with the untreated sample.The sample treated at an induction temperature of 520 ℃ has severe surface damage,exhibiting large-scale spalling,which is called spalling pits.The spalling pits are round with a size of 30-40 μm,and the depth increases by 82.7％compared with the untreated sample.As the temperature further decreases to 490 ℃,the surface damage becomes more severe,and the spalling pits continue to enlarge and interconnect,forming a larger damaged area.The depth of the spalling pits reaches 9.28 μm,which is increased by 132.6％compared with the untreated sample.The induction temperature also has a significant influence on the crack propagation mechanism(Fig.8).For the untreated sample and the sample treated at 550 ℃ induction temperature,cracks initiate from the material surface and gradually propagate towards the interior.However,for the samples treated at 520 and 490 ℃,cracks initiate mostly at subsurface positions and gradually propagate towards the material surface and deeper positions.This phenomenon is attributed to the plastic deformation that occurs in the material during the rolling contact process,resulting in the formation of a plastic deformation layer as the number of cycles accumulates.In the untreated sample and that treated at 550 ℃,surface cracks propagate along the direction of plastic flow toward the interior.As the induction temperature decreases,the martensite structures in the strengthening layer continue to increase and gradually dominate.The material's resistance to plastic deformation at the surface improves,resulting in a smaller growth space for surface cracks.The poorly plastic martensite becomes susceptible to cyclic shear forces and gradually fractures,leading to the initiation of brittle cracks.Conclusions A U75V steel surface was subjected to strengthening using a laser solid-state phase transformation process,and the effects of induction temperature on its rolling contact fatigue performance were investigated.The following conclusions were drawn:First,the induction temperature has a significant influence on the morphology of surface damage.Higher induction temperatures result in reduced damage in terms of both quantity and size,whereas lower temperatures result in extensive material delamination and aggravated damage.Second,the induction temperature significantly affects the initiation position of cracks.Lower induction temperatures cause cracks to initiate from the subsurface positions of the samples.Finally,the induction temperature has an important impact on the mechanism of rolling contact fatigue damage.At higher temperatures,fatigue pitting damage is the dominant form,whereas,at lower temperatures,fatigue spalling damage becomes the main form,resulting in extensive delamination of the material surface and the risk of overall detachment of the hardened layer.In summary,the selection of induction temperature plays a crucial role in the improvement of the rolling contact fatigue performance of the material.
laser solid-state phase transformationU75V railfatigue damageinduction temperature
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